Conventionally, as an antenna for mobile apparatus in mobile communication, an external type antenna such as a rod antenna and a helical antenna is provided protruding from the housing of a mobile terminal.
However, the protruding antenna has drawbacks that it interferes with handling and is subjected to breakage. Moreover, it is not desirable from the aspect of design. Therefore, built-in antennas are apt to substitute for the external type antenna.
FIG. 1 shows the general composition of a mobile terminal (cellular phone) equipped with a built-in antenna. The cellular phone is composed of a printed-circuit board 101, a liquid crystal 102, an RF circuit 104 housed in a shield case 103 on the back of the board 101, an antenna 105 and a battery 106. These components are compactly housed in a housing 107.
As shown in this composition, the antenna 105 is generally incorporated at the upper portion of housing because its user holds the lower portion of housing 107 in phone call. Thus, the periphery of antenna 105 is kept not to be shielded with the hand of user and a good line for calling can be secured. The built-in antenna is able to solve all the drawbacks of external type antenna and to provide a cellular phone with good function and design.
FIG. 2 shows the general structure of built-in antenna 105. The built-in antenna 105 is composed of an insulative support body 108 with polyhedral portion formed on the top, and a metal conductor 109 that is formed by pressing and bending a copper plate etc. to have a shape to be tightly fitted on the support body 108. These are integrated using an adhesive agent etc. 110 is a slit formed in the metal conductor 109 to adjust the resonance frequency, 111 is a feeder terminal and 112 is a ground terminal.
By the way, it is required that the metal conductor 109 has a volume as much as possible to secure a wider band characteristic since there is a problem that built-in antennas have a band characteristic narrower than external type antennas. Therefore, the shape is made to be polyhedral rather than flat plane. Namely, as shown in FIG. 2, the top of support body 108 is composed of a flat plane 113 and inclined planes 114, 115 extending downward from the flat plane 113, and the metal conductor 109 is formed into a shape to be fitted thereto by bending. The metal conductor 109 thus formed has a volume greater than that of flat plate type, and the band characteristic thereof is enlarged by that much.
FIG. 3 shows a built-in antenna that is suggested to have a further enlarged band characteristic.
The built-in antenna in FIG. 3 is composed such that the support body 108 and metal conductor 109 shown in FIG. 2 are extended as shown by double-dashed chain lines in FIG. 3. Surfaces (116) to (118) are added both to the support body 108 and metal conductor 109 and, therefore, the volume of metal conductor 109 is further increased to further enlarge the band characteristic. The feeder terminal 111 and ground terminal 112 are, of course, provided on the surface (117).
The polyhedral shape thus formed is desirable not only for enhancement in antenna property but also for housing easily the antenna in the upper portion of housing 107 that is formed, as shown in FIG. 1, tapered to meet the demand of design. Thus, the built-in antenna thus composed offers a good utility to various mobile terminals such as a cellular phone.
However, in the built-in antenna shown in FIG. 3, it is difficult to form the four surfaces (117), (118), 119 and 120 of metal conductor 109 by bending without having any strain at, e.g., the corner A of flat plane 113. Therefore, when the metal conductor 109 is mounted on the support body 108, excessive part of copper plate to be generated by bending may form uneven region at particularly bended portions among the surfaces (117), (118), 119 and 120. Such an uneven region may be also formed at the opposite corner B.
As a result, there may occur a deviation in band characteristic between devices due to the uneven region formed in the vicinity of the bended portions of metal conductor 109. Further, when mounting the metal conductor 109 on the support body 108, there may occur a gap in the vicinity of bended portions. Therefore, the metal conductor 109 may be not stably held by the support body and, therefore, the band characteristic may be affected adversely.